A speed reducer having a planetary gear mechanism is used for reducing the speed of rotation of a motor, for example. As one type of the planetary gear mechanism, there is widely known a planetary gear mechanism having a first shaft, an externally toothed gear mounted via an eccentric body provided on the first shaft such that the gear is eccentrically rotatable relative to the first shaft, an internally toothed gear which the externally toothed gear internally contacts and meshes with, and a second shaft coupled to the externally toothed gear via a means for transmitting only the rotational component of the externally toothed gear. As a specific example of this type of planetary gear mechanism, a cycloid differential planetary gear mechanism is known.
The speed reducer using the cycloid differential planetary gear mechanism achieves a large speed reduction ratio with one reduction stage, and operates with a high efficiency owing to a high contact ratio, as compared with an planetary gear mechanism having general gears of an involute tooth profile. On the other hand, the speed reducer of the cycloid type has a complicated mechanism for taking output out of eccentric oscillating rotation, and is likely to be available at a high cost.
A general cycloid differential planetary gear mechanism causes an epitrochoid externally toothed gear to internally contact a pin gear as an internal gear and eccentrically oscillate, so as to provide output via inner pins. In order to cancel out an imbalance due to the eccentric arrangement, an externally toothed gear having the same structure as and opposite in phase to the above-indicated externally toothed gear is added, or two pieces of externally toothed gears having the same structure and shifted in phase by 120° from each other in terms of the direction of displacement of the center are added (see Patent Document 1).
However, the known cycloid differential planetary gear mechanism has a problem that it cannot provide a large speed change ratio (reduction ratio). Therefore, in order to achieve an even larger reduction ratio (e.g., 100 or larger) with the speed reducer having the known mechanism, another stage of speed reduction mechanism or speed reduction unit must be added the upstream side of the input shaft or the downstream side of the output shaft, resulting in a significant increase in the cost and an increase in the installation space.
More specifically, the internal gear needs to be constituted by a large number of pins, so as to provide the epitrochoid gear. In the case of a small-sized speed reducer, in particular, the number of pins that can be placed on the inner periphery needs to be large. However, the diameter of each pin needs to be reduced so as to increase the number of pins. It is practically impossible or quite difficult to make this choice, eventually making it impossible to realize a structure that provides a large reduction ratio.